Furnace for producing single crystals for transistors



A. MARK 2,789,153

FURNACE FOR PRODUCING SINGLE CRYSTALS FOR TRANSISTORS April 16, 1957Filed Feb. 28-, 1956 INVENTOR. W M

ATTORNEY United States Patent Oflice 2,789,153 Patented Apr. 16, 1957FURNACE FOR PRODUCING SINGLE CRYSTALS FOR TRANSISTORS Albert Mark, TomsRiver, N. J.

Application February 28, 1956, Serial No. 568,293

3 Claims. (Cl. 13-31) This invention relates to electric furnaces.

The furnace of this invention is the only one, so far as known, whichdoes not add impurities to a melt.

The hereinafter disclosed furnace is especially used to melt materialswhich should not be in the least contaminated, such materials forexample being silicon and germanium, used to produce transistors, itbeing known that an ultra high degree of purity should be maintained inthe materials used for such purpose. In fact, it is most desirable thatthere be no contaminants which are detectable spectroscopically, and thepresent furnace meets this requirement, and consistently so. Consistencyor uniformity of the product is highly important, and is not obtained byother furnaces.

The furnace is also adapted to be used with pulling mechanism, in theformation of a single crystal of silicon or germanium, for example,single crystals being essential, as known, for the making oftransistors, and success in obtaining a single crystal being largelydependent on maintaining high purity of the molten material in afurnace. Moreover, the higher the purity, the better the lifetimecharacteristics of transistors.

Another advantage of the furnace, hereinafter described and claimed, isthat it avoids the present radio frequency induction heating system, nowmuch used, which is undesirable not only because it produces somecontamination, but also because it requires very elaborate, andtherefore very expensive, control equipment, costing about $25,000 foruse with a very small furnace. The R. P. system of heating was resortedto in an attempt to avoid contaminating the melt, but has beenunsuccessful because the structure of such furnaces has not beencorrect, notwithstanding the great amount of effort, time and money,expended on them in view of the importance of transistors and thedesired extremely high purity.

The drawings illustrate the invention, as required, and in these:

The single figure is a longitudinal section of a furnace, partly brokenaway.

Referring to the drawing for a more detailed description thereof, thenumeral 3 indicates the furnace in general. The bottom 4 of the furnaceis elevated on legs 6 and is secured thereto, and may be of metal orother suitable material. An outer cylindrical tube 7 rests on the bottom4 and is hermetically sealed thereto, and a top or cover 8 for the tubeis also so sealed. Said tube, with its cover, is preferably a quartz,SiO2, tube, but is not necessarily of that material. An inner tube 10 ofquartz, SiOz, also rests on the mentioned base 4 and is hermeticallysealed thereto, and is spaced from the outer tube 7. It is here to beremarked that the term quartz, as used in this specification, includesany form of SiOz which forms an impervious tube or enclosure.

Said inner tube 10 is formed with a cylindrical reentrant well 12 in itsupper portion, the well also being of quartz, and in this well acylindrical quartz, SiOr, crucible 14 is disposed, being spaced fromsaid well laterally and at its bottom. The crucible has an enlargedupper end 15 which rests on top of the inner tube 10, being therebysupported in proper position, and it has a reduced opening 17 at itsupper end to receive a charge to be melted, and also to withdraw acrystal from the molten charge.

A heater 20, of helical form, surrounds the well 10 and extends belowit, being adjacent to the well but spaced slightly from it. The heateris preferably of graphite, but may be of other high melting pointmaterial, as tantalum or molybdenum for example. The heater is supportedby quartz-enclosed conductors 23 and 24 which may beof tantalum or otherhigh melting point material, and which are secured respectively to theupper and lower end of the helix, said conductors being connected to,and supported by, bolts 26 and 27 respectively, which pass thru thebottom 4 of the furnace, to be connected to a source of power. Thedrawing also shows a thermocouple 29, which includes a quartzenclosing-tube and which contacts the bottom of the well 12 so that thetemperature may be known and controlled, and is supported by a clamp 30which is held in position by rods 32 and 33, the lower ends of theserods being embedded in the bottom 4 of the furnace. The thermocouplepasses thru a heat-reflecting tantalum shield 35, the latter beinghorizontally disposed, and spaced a little below the lower end of theheater, being provided to reflect and therefore conserve the heat of thelatter.

Means are provided for exhausting air and other gases and vapors fromthe furnace, and preferably for exhausting independently from the spaceinside and outside the inner tube 10; such means being shown ascomprising pipes 38 and 39, which are connected to a common pipe 40which leads to a vacuum pump. it will be noted that pipes 38 and 39 areprovided respectively with shut off valves 40 and 41 and that pipe 38leads into the bottom of inner quartz tube 10, and also that pipe 39leads into the lower end portion the outer tube '7.

The drawing further shows pulling means extending thru the top 8 of theouter tube 7 and into the quartz crucible 14, this means comprising avertical rod 45 slidably passing thru the top or cover 8, which may bethru a seal, and is intended to be simultaneously rotated and pulledoutwardly, when attached to a "seed crystal of the same material as thatin the crucible. The seed crystal is lowered into the molten material inthe crucible and the rod 45 is then slowly pulled outwardly, whilerotating. Thereby the molten material crystallizes on the seed crystaland thus produces a large crystal.

The crucible wall, and also the wall of the inner tube 10, is about $1inch thick, and the crucible inner diameter about 1 inch. The spacebetween the bottom of the crucible and the bottom of the well 21 isabout inch, While the lateral space between the crucible and the well isabout /a inch. The given dimensions are suitable for a furnace about 3times the size shown in the drawing, and the usual charge in thecrucible may be from 50 to 60 grams of silicon. For such charge, about60 amperes of current are sent thru the heater.

It will be seen from the foregoing description that the heater 20 isenclosed by impervious walls, and is isolated from the crucible and itscontents, whereby particles and gases from the heater may notcontaminate the crucible charge, the heater, running as it does at ahigh temperature, being the main source of such contamination, and itwill be appreciated by those skilled in the art that the construction ofthe furnace is otherwise such as to eliminate contamination.

Before running the first melt, a new furnace should be thorolydegasified by running the vacuum pumps with the furnace heated.Subsequently, with a charge of silicon in the crucible, the furnace ispumped down to a vacuum of about 10' mms. After about a half hour at C.and held there for about 2 hours, with the vacuum held as stated. Thetemperature is then increased to about 1420 C., which is about themelting point of silicon, and the vacuum brought to 1-0" '-mrns. ofmercury, the charge being held under these conditions for an hour ormore. The process stated serves-to purify the silicon by removing gases,and impurities volatile at the statedtemperatures, which are removedfrom the furnace by the vacuum condition, and the process also serves tore move gases and'some impurities from components of the furnace itself.A similar process is followed for other metals, as germanium, forexample, regard being had for the melting point of the metal.

For more precise heating, the tantalum reflector is replaced with agraphite heater which is made with a} spiral base. Provision is thenmade to position the shorter of the two power electrodes to the centerof the graphite base. The molten charge is therebyalso heated directlyfrombelow as well as from the sides. This reduces the 7 temperaturegradients in the molten mass since the charge is heated evenlythroughout.

What is claimed is:

1. An electric furnace comprising an upright quartz tube, including inits upper portion a reentrant well, a

quartz crucible within said well and spaced above the bot nected to theends of said heater and supporting the latter, quartz shields aroundsaidconductors, atant-alumheatrefiector below and adjacent to the lowerend of said heater, a second tube around and spaced from said quartztube and crucible, and means for exhausting gases and vapors from thefurnace.

2. An electric furnace comprising an upright quartz tube including inits upper portion a reentrant well, a quartz crucible within said well,a heater around. and adjacent said well and adapted to. be electricallyheated, at second tube around said quartz tube and spaced therefrom, andmeans for independently exhausting gases and p r rom a nd the e l anwithin he quar z tu and from within the well and crucible and the spacebetween the quartz and second tubes.

3. in an electric furnace, a quartz tube comprising in its upper portiona reentrant well, a crucible Within said well and spaced above thebottom and from the sides of said well, and aheat'er around andadiacent'said well and within said quartz tube and sealed nit airtight from saidcrucible, and: a second airtight tube around said quartz tube'and saidcrucible, both of said tubes being exhaustible of air.

References Cited in the file of this patent UNLTED STATES PATENTS

1. AN ELECTRIC FURNACE COMPRISING AN UPRIGHT QUARTZ TUBE, INCLUDING INITS UPPER PORTION A REENTRANT WELL, A QUARTZ CRUCIBLE WITHIN SAID WELLAND SPACED ABOVE THE BOTTOM, AND FROM THE SIDE, OF THE WELL, A HELICALGRAPHITE HEATER ENCLOSED WITHIN SAID TUBE AND DISPOSED AROUND ANDADJACENT SAID WELL AND ADAPTED TO BE ELECTRICALLY HEATED CONDUCTIVELY,TANTALUM ELECTRICAL CONDUCTORS CON-